1. Field of Invention
The present invention relates to fabrication of a micro-electro-mechanical systems (MEMS) device. More particularly, the present invention relates to a technology of releasing diaphragm of a MEMS device.
2. Description of Related Art
A MEMS device, such as MEMS microphone, basically includes two main structure parts, one of which is a diaphragm and another of which is a backplate. The diaphragm can be moved due to external factors.
Taking MEMS microphone as an example, the backplate has venting holes to connect to a cavity, which usually receives acoustic signal. The variance of the air pressure from the acoustic signal would cause the vibration of the diaphragm, so that the diaphragm can sense the acoustic signal. When the diaphragm is vibrated by the acoustic signal, the distance between the diaphragm and the backplate, which is steadily still, would also be changed correspondingly, and a variance of capacitance between the diaphragm and the backplate in response to the acoustic signal can be converted into electric signal for subsequent use by the electronic system on which the MEMS microphone is implemented.
As the operating mechanism of the MEMS device, the sensitivity is depending on how the diaphragm responds to acoustic signal or any force factor asserted on the diaphragm. In semiconductor fabrication procedure, the diaphragm is usually formed with the dielectric layer, which may involve multiple sub dielectric layers as known in the art. At the stage of semi-finished product before the diaphragm is released, the diaphragm is embedded in the dielectric layer. It usually needs a wet etching process to etch a portion of the dielectric material, so as to release the diaphragm, and therefore allow the diaphragm to sense the detected force factor, such as the air pressure of the acoustic signal.
The sensitivity of the MEMS device is quite related to the capability of shift of the diaphragm. However, during the wet etching process, the etchant relative to the silicon substrate having the cavity is hydrophobic or repellent, so a bubble may occur on the surface of the substrate. The bubble may cover a portion of surface of the dielectric layer, and may result in poor condition for the wet etching process on the dielectric material.